Enable throttleing of HttpClient per host, with default being 6.

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index bea663db3a098c333b542b439f08d77d9e0d1b6a..5d1d53598e03a37bcf3a8731819c1c62738df1f8 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -1522,6 +1522,7 @@ class _ConnectionTarget {
   final bool isSecure;
   final Set<_HttpClientConnection> _idle = new HashSet();
   final Set<_HttpClientConnection> _active = new HashSet();
+  final Queue _pending = new ListQueue();
   int _connecting = 0;
 
   _ConnectionTarget(this.key, this.host, this.port, this.isSecure);
@@ -1530,7 +1531,7 @@ class _ConnectionTarget {
 
   bool get hasIdle => _idle.isNotEmpty;
 
-  bool get hasActive => _active.isNotEmpty && _connecting > 0;
+  bool get hasActive => _active.isNotEmpty || _connecting > 0;
 
   _HttpClientConnection takeIdle() {
     assert(hasIdle);
@@ -1541,6 +1542,12 @@ class _ConnectionTarget {
     return connection;
   }
 
+  _checkPending() {
+    if (_pending.isNotEmpty) {
+      _pending.removeFirst()();
+    }
+  }
+
   void addNewActive(_HttpClientConnection connection) {
     _active.add(connection);
   }
@@ -1549,12 +1556,15 @@ class _ConnectionTarget {
     assert(_active.contains(connection));
     _active.remove(connection);
     _idle.add(connection);
+    connection.startTimer();
+    _checkPending();
   }
 
   void connectionClosed(_HttpClientConnection connection) {
     assert(!_active.contains(connection) || !_idle.contains(connection));
     _active.remove(connection);
     _idle.remove(connection);
+    _checkPending();
   }
 
   void close(bool force) {
@@ -1577,6 +1587,15 @@ class _ConnectionTarget {
       client._updateTimers();
       return new Future.value(new _ConnectionInfo(connection, proxy));
     }
+    if (client.maxConnectionsPerHost != null &&
+        _active.length + _connecting >= client.maxConnectionsPerHost) {
+      var completer = new Completer();
+      _pending.add(() {
+        connect(uriHost, uriPort, proxy, client)
+            .then(completer.complete, onError: completer.completeError);
+      });
+      return completer.future;
+    }
     var currentBadCertificateCallback = client._badCertificateCallback;
     bool callback(X509Certificate certificate) =>
         currentBadCertificateCallback == null ? false :
@@ -1604,6 +1623,10 @@ class _ConnectionTarget {
           addNewActive(connection);
           return new _ConnectionInfo(connection, proxy);
         }
+      }, onError: (error) {
+        _connecting--;
+        _checkPending();
+        throw error;
       });
   }
 }
@@ -1625,6 +1648,8 @@ class _HttpClient implements HttpClient {
 
   Duration get idleTimeout => _idleTimeout;
 
+  int maxConnectionsPerHost;
+
   String userAgent = _getHttpVersion();
 
   void set idleTimeout(Duration timeout) {
@@ -1794,7 +1819,6 @@ class _HttpClient implements HttpClient {
   // Return a live connection to the idle pool.
   void _returnConnection(_HttpClientConnection connection) {
     _connectionTargets[connection.key].returnConnection(connection);
-    connection.startTimer();
     _updateTimers();
   }